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A study of global water resources shows that one third of the worlds population is currently living in regions classified as 'water stressed' and predicts this figure will grow to half the world's population by 2025.

Associate Professor Kenneth Strzepek of the civil, architectural and environmental engineering department at the University of Colorado (UC), and his colleagues have used sophisticated computer models and geographical information systems to look at river basins around the world and identify those that are the most stressed. Some of the "hot spots" include China's Yellow River basin, Africa's Zambeze River basin, the Syr Darya and Amu Darya River basins leading to Russia's Aral Sea and the Colorado River basin.

His research is part of a background analysis for the World Water Commission's World Water Vision for the 21st Century report. The commission is a government and privately funded organization seeking global solutions to water problems.

Strzepek has been working on the project with UC graduates Alyssa Holt of the Stockholm Environmental Institute and Jeff Bandy of the University of Leuven in Belgium.

The modeling tools have allowed Strzepek and his colleagues to look at the vegetation, soils and climate from the headwaters to the mouths of the world's major river systems in 25 mile-square chunks to model runoff and streamflow. They also have been able to use past temperature and precipitation data to reconstruct runoff and streamflow data for major river basins going back 100 years.

Today, 70 percent of the world's freshwater withdrawn by humans is used to irrigate crops, he said. Unfortunately, much of the irrigation water takes up pesticides, herbicides and salts from cropland soil and returns to the river system, polluting the water and adversely affecting humans and the environment.

"In the Nile Delta in Egypt, water quality is a major problem for human and agricultural use due to upstream pollution from agricultural, industrial and municipal uses," Strzepek said. "Similar situations are found in other river systems like the Indus River in Pakistan and the Yellow River in China."

While more dams provide more water for humans, they cause environmental impacts, he said. Researchers have set a goal that no more than 40 percent of a river basin's water should be diverted for human use in order for the environment to be adequately protected.

"In the Colorado River basin, however, 100 percent of the water is used," Strzepek said. While the average flow in the basin is 15 million acre-feet per year, only 1.5 million acre feet is delivered annually to Mexico and only a trickle remains as the Colorado River enters the Gulf of California in Mexico.

"The goal of the World Water Commission is to make water everybody's business in the 21st century and to see that the needs of nature and humans are met as we pursue the task of sustainable economic growth," Strzepek said.